This invention generally concerns control systems for document feeders, and is specifically concerned with a system and method for dynamically controlling the rate that a document feeder delivers documents to the scanning station of an optical scanner such that throughput for a given processing bandwidth is maximized.
Optical scanners for scanning and recording optical data present on documents are well known in the prior art. Such optical scanners typically include a scanning station having an optical reading element, a light source, and a platen formed from glass or transparent plastic for maintaining a document in a flat position as it is moved across the line-of-sight of the optical element by an assembly of belts and rollers. In some scanners, the documents stay stationary in the platen while the optical reading element is moved. Either type of scanner includes a document feeder for regularly feeding a train of documents into the document-conveying rollers and belts of the station. In operation, optical data from the documents is transmitted from the optical reading element to one or several buffer memories, which temporarily stores the data for further processing, typically includes the compression of the data and permanent storage thereof onto a magnetic disk.
Because such scanners are often used to make permanent records of important documents, it is important that they operate in such a manner that no data is lost. Such data loss could occur if the capacity of any one of the buffer memories were overloaded as the document was transported past the scanning element. To avoid such a scenario, the speed of the document feeders is deliberately limited to a speed in which the documents containing the highest density of optical information could be delivered to the scanning station and read thereby without overloading the available buffer memory capacities. Unfortunately, in limiting the transport speed to such xe2x80x9cworst casexe2x80x9d scenario documents, the inventors have observed that the scanner operates at a speed which is slower than necessary for most of the documents contained within a particular scanning run or batch.
Clearly, there is a need for a control system for an optical scanner that does not limit the delivery speed of the document feeder to those documents having the highest density of optical data. Ideally, such a system would operate the document feeder at the fastest rate consistent with the amount of optical data present on a typical document contained within a scanning run, and would automatically pause or slow down the feeder when documents having a higher than normal density of optical data were read by the scanning station without the need for operator intervention. Finally, such a system should be simple and inexpensive to construct and install, easily retrofittable on existing scanners, and should operate reliably without loss of optical data.
Generally speaking, the invention is both a control system and method for an optical scanner of the type having a mechanism for feeding documents to a scanning station that overcomes the throughput limitations associated with prior art scanners and meets the aforementioned needs in the art. To this end, the control system of the invention comprises a plurality of buffer memories for temporarily storing optical data from documents processed through the scanning station, and a control circuit connected to the buffer memories and the document feeding mechanism for transmitting a command to change a delivery rate of the document feeding mechanism upon a determination that a threshold capacity of any of the buffer memories has been exceeded or is about to be exceeded.
The control system may include a buffer monitor circuit for determining whether any one of the plurality of buffer memories becomes filled to the threshold capacity, which may be indicative of an xe2x80x9calmost fullxe2x80x9d state of each of the buffer memories. This xe2x80x9calmost fullxe2x80x9d threshold capacity may be pre-selected, and the command to change the delivery rate of the document feeding mechanism may be a pause command. A second threshold capacity may also be pre-selected for each of the buffer memories which is less than the xe2x80x9calmost fullxe2x80x9d threshold for each and which indicates that the particular buffer memory is xe2x80x9csufficiently emptyxe2x80x9d to receive additional optical data. The buffer monitor circuit may generate a first set of signals indicative of whether the xe2x80x9calmost fullxe2x80x9d threshold of any of the buffer memories has been exceeded, as well as second set of signals indicative of whether or not the capacity of the buffer memories is below the xe2x80x9csufficiently emptyxe2x80x9d threshold. The first and second signals may be transmitted to first and second gate circuits, and the first gate circuit may generate a pause command upon receipt of a signal that any of the buffer memories is above the xe2x80x9calmost fullxe2x80x9d threshold, while the second gate circuit may cancel the pause command upon receipt of signals from each of the buffer memories that all (or selected) of the memories are below the xe2x80x9csufficiently emptyxe2x80x9d threshold.
The capacity of each of the buffer memories may also be pre-selected by electronically changing the portioning between the various memories. In other words, one buffer memory may be expanded in capacity at the expense of another prior to running a particular batch of documents. This feature is particularly useful in efficiently allocating buffer memory resources when, for example, there is typically less information on the back side of the document than the front sides, or only grayscale images are needed rather than color images.
In an alternate embodiment of the system, the buffer monitor circuit and gate memories may be replaced with a processor which simultaneously monitors the remaining capacity of each of the buffer memories, as well as the trends associated with the density of optical data contained in the documents being scanned, and individually determines, on a floating basis, both an xe2x80x9calmost fullxe2x80x9d threshold and a xe2x80x9csufficiently emptyxe2x80x9d threshold for each of the buffer memories. In such an alternate embodiment, the implementation of the changing of the delivery rate of the document feeding mechanism is not limited to the generation and cancellation of a pause command, but also includes a speeding up or slowing down of the delivery rate which is dependent upon (at least in part) extrapolated trends in the density of optical data in the documents being processed by the scanning station.
In both embodiments of the system, document throughput is substantially increased by adjusting the delivery rate of the sheet feeder to a maximum rate consistent with the density of the optical data on the documents actually being scanned.